1. Field of the Invention
The present invention relates to improvements of a radio-frequency tag communication system including a radio-frequency tag communication device configured to effect radio communication with a radio-frequency tag, for writing and reading information on and from the radio-frequency tag.
2. Description of the Related Art
There is known a radio-frequency identification system (an RFID system) including small-sized radio-frequency tags (transponders) storing predetermined information, and a radio-frequency tag communication device (interrogator) arranged to read desired information from the radio-frequency tags in a non-contact fashion. The radio-frequency communication device of this RFID system is capable of reading out the information from the radio-frequency tags, by radio communication with the radio-frequency tags, even where the radio-frequency tags are soiled or located at invisible places. For this reason, the RFID system is expected to be used in various fields such as management and inspection of commodities.
In the field of such a radio-frequency tag communication device, there is known a technique for changing the directivity of communication of the radio-frequency tag communication device with a communication object in the form of the radio-frequency tag. JP-2003-283411A discloses an example of the radio-frequency tag communication device configured according to such a technique, which is provide with an array antenna device including a plurality of antenna elements, and an adaptive processing portion configured to multiply received signals received by the respective antenna elements, by respective weights. The adaptive processing portion configured to multiply the received signals by the adequate weights can suitably control the directivity of reception of the array antenna device, permitting adequate radio communication with the desired radio-frequency tag.
However, the prior art technique described above does not permit setting of an angular range of direction in which the directivity of communication is controlled, that is, does not permit setting of an effective range of communication. Accordingly, the direction of communication must be changed over an angular range unnecessarily wide as to cover a directional range in which the radio-frequency tag is unlikely to exist. Therefore, the prior art technique suffers from a drawback of requirements for an unnecessarily long time and an unnecessarily large amount of power consumption. Thus, there has been a need of developing a radio-frequency tag communication system wherein the radio-frequency tag communication device is capable of adequately setting its control range of directivity of communication.